Allow me to share my little knowledge from my occupation.
It is very hard/difficult to determine which temp sensor Toyota uses for the main temp reading. Such as which one does the dash light use and also the SG. I know the ECM uses both for varying reasons.
The idiot light is programmed to come on at 240. It should be pretty accurate but I can't verify that and am not willing to push my fluid that high to see!
The highest I have seen on my SG is 202. That was during a 90 degree day while four wheeling in some very slow and aggressive trails. In this situation the engine compartment gets very hot and with little air being pulled across the trans cooler it is to be expected.
While on the road I have noticed that they temp will start rising without fail if I do not keep it in 5th gear/OD. At a stop light if you shift into N it will start climbing as well. This is because the pump is not moving the fluid as rapidly.
Ambient temps have a huge difference on my trans temps. Yes, this is pretty common sense but it REALLY makes a bigger difference than I would have imagined. I am assuming it's because the cooler is the most forward and gets most of the air flow. Remember that at idle it is just the opposite. The engine fan air flow has to pull air from 1st the radiator, then condenser and finally the trans cooler. If your cooling fins on any of the 3 are restricted by dirt, leaves, bugs etc it makes it that much harder.
In the HVAC business they use a product for cleaning the condenser coils outside of the house or business. I have some and use it on my condensers and also my vehicles. This stuff is absolutely amazing. Especially if you use it on your home condensers you will freak when you see what this stuff cleans out. I bought a gallon jug of it a few years ago and can't remember where I got it. HD and Lowes sells it in a aerosol can. I like the fact I can put it in a spray bottle. Here is a link from Amazon.
Nu-Calgon 4291-08 Nu-Brite Coil Cleaner - 1 Gallon, Blue: Ac Condenser Cleaner: Amazon.com: Industrial & Scientific
One more thing. If you ever think or know that your fluid is getting dangerously hot and you pull over do NOT ever turn the car off. All the manuals will tell you to pull over and shift to park and allow it to cool down while idling. However, I would leave it in D with my foot on the brake. As I stated above my temp always rises after I shift to P from running pretty hard.
Ya'll have posted some good ideas and thoughts. I figured I would share mine as well.
CSX/Chris~If I ever am able to determine exactly which sensor they use you will be the 1st to know! These 2 sensors are very close to each other internally so it's not like they are on opposite ends of the system from each other.
